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Intermittent acceleration issue

Discussion in 'The Garage' started by Mitch81special, Sep 16, 2020.

  1. This one is a head scratcher. The bike sputters and dies, almost as if its out of fuel or in too low a gear (not the problem). but this only happens in gear, it idles fine and will rev normally in neutral. It's also an intermittent problem, has happened 3 times in the last 3 months. Twice just as I was leaving and once in the middle of a ride. Shutting it off seems to fix it. Thoughts?
    Last edited: Sep 16, 2020
  2. hovel

    hovel ol' stupid

    Fuel flow. Idling and no load revs don't require fuel like putting it under a load. Maybe the tank vent is clogged. Maybe a float valve is intermittently hanging up. Try rapping on a float bowl with the handle of a screwdriver or something similar.
    Mitch81special likes this.
  3. Thanks, I didnt know idling/no load would use less fuel. That makes sense.
  4. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    In neutral, no loads, the manifold pressures will be low, producing low compression pressures, requiring low demands of the ignition system.

    In gear, with loads, the manifold pressures will be higher, producing higher compression pressures, stressing the ignition system.

    Could be not enuff spark *oomph*.

    One diagnostic trick is to reduce the plug gaps, maybe down to about 0.015".
    See if it makes any difference...
    Paul Sutton and Mitch81special like this.
  5. Thanks I'll try that
    Paul Sutton and TwoManyXS1Bs like this.
  6. Thanks, I just checked my plug gaps and they were .035" iv reset it to .015
    Hopefully that is the issue.
    Last edited: Sep 20, 2020 at 12:38 AM
  7. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    On a temporary basis.....
    TwoManyXS1Bs likes this.
  8. bikefreakjon

    bikefreakjon XS650 Addict

    Spark gap should be .032". Don't change that.

    Check spark with a timing light while riding, if the light flicker whenever there's a problem then it's ignition.

    My money is on a fuel issue though
  9. 5twins

    5twins XS650 Guru Top Contributor

    Yes, I don't go bigger than .032" on either one of mine, even though one has a better aftermarket coil. If the ignition is stock, it might struggle to fire .035" reliably all the time. When I still had points on my '78, I ran .028" gaps. Factory spec for the later models is .7 -.8 mm (.0276" - .0315"), so even .032" is a bit above that. As a happy median, you might try .030".
  10. Iv got an 81 with I think stock TCI. Iv got it set to .015" just to see if that fixes my issue as suggested by TwoManyXS1Bs. But as it's kind of an intermittent issue I'd have to run it for awhile waiting to see if if happens again. What kind of problems can running it with to small a gap create?
  11. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Mitch81special, it's just a simple, cheap, quick diagnostic test.
    I like simple, cheap, quick.

    Reducing the plug gap simply reduces the voltage required to spark the gap. Helps to diagnose for weak ignition system, like weak/overheated/cracked/shorting coils, failing ignition resistances (plug, cap, plug wires), ...etc.

    If the engine runs better, then you have a direction for your next tests.

    It's not a fix, but can be used as an emergency fix to get home.

    After the proper fix, always set plug gaps accordingly.

    For more drivel, do a forum search on "plug gap", posted by "TwoManyXS1Bs"...
    Mitch81special likes this.

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